Introduction

The atmosphere of the Earth is opaque to most wavelengths of
light in the infrared, in the ultraviolet and in the
X-rays. Due to this fact observation of celestial objects in
X-rays (in the range of energies from 0.5 to 20 keV) must
be carried out outside the atmosphere at altitudes greater
than 100 kilometers, namely from space. The development of
X-ray astronomy had therefore to await the development of
rocket and satellite borne instrumentation.

The first observations of solar X-rays were carried out in
1948 from captured German V-2 rockets by the group at the
Naval Research Laboratories (NRL) led by Herbert
Friedman. The first observation of extra- solar x-ray
sources was obtained in 1962 from an Aerobee rocket by the
group at American Science and Engineering (AS&E) led by
Riccardo Giacconi.

In the fifty years since then x-ray astronomy has grown to be an
important branch of astronomy on par with optical, infrared and
radio. The sensitivity of X-ray observations has increased in this
period by more than 10 billion times, an improvement equal to that
achieved in optical astronomy from the naked eye to the 10 meter
telescopes, which occurred over 400 years.

This improvement in sensitivity of the instrumentation has allowed the
study of X-ray emission from all types of celestial objects from
planetary magnetospheres to stars, to the most distant quasars in the
universe. As described in what follows X-ray observations have
permitted the discovery of previously unknown celestial objects and
states of matter.

The importance of these studies is due to the prevalence of high
energy phenomena in the formation and dynamic evolution of stars and
galaxies. High energy phenomena are events such as explosive processes
where particles are accelerated to relativistic energies, or processes
resulting in the heating of plasmas to extremely high temperatures
(100 million degrees).

All of these phenomena are copious emitters of X-rays which
are the lowest energy photons more energetic than UV (and therefore
the most numerous) that can reach us from cosmological distances.
Furthermore X-rays (as opposed to gamma rays) can be
focused with grazing incidence telescopes and it is the development of
these telescopes which has permitted the rapid improvement in
sensitivity noted above.

In the last decades X-ray observations have become a necessary and
fundamental tool to study the Universe.

Instrumentation

Observations in X-ray astronomy consist normally in the detection of
individual photons. In the 50s and 60s, thin window Geiger or
proportional counters were used. The Geiger counters detected each
photon absorbed in their gaseous volume, but gave no information as to
its energy; in the proportional counters the electrical signal
produced by each photon was proportional to its energy. Geiger
counters and proportional counters were used in the discovery flights
of NRL and AS&E, in the first X-ray observatory “UHURU” in 1970, and
in the “High Energy Astronomical Observatory-A” in 1977. The
sensitivity of these detectors was limited by the background and it
could therefore only increase with the square root of the area. The
100 square foot detector on HEAO-A was only 7 times
more sensitive than the 2 square foot detector of UHURU.

The breakthrough in X-ray astronomy occurred with the development of
X-ray grazing incidence telescopes by the AS&E group in Cambridge,
Massachusetts. In particular, Leon Van Speybroeck and Giuseppe Vaiana
of that group succeeded in designing and building X-ray telescopes of
increasingly high angular resolution
(see Giacconi et al. 1969).
In 1968 the first high resolution picture of the Sun (5 arc seconds) was obtained in a rocket
flight by the AS&E group. In 1973 the same group achieved equal
resolution in a set of photographs extending over a full solar
rotation with a 30 cm diameter telescope on SKYLAB, the first manned
Space Station.

Figure 2: Progress in sensitivity in 40 years of X-ray astronomy. Number of sources per square degree, N(>S), brighter than a given flux, S, for the 0.5–2 keV band. Courtesy of G.Hasinger.

Imaging X-ray telescopes of increasing size and resolution were flown
for extra-solar X-ray astronomy on the “EINSTEIN” observatory by
Giacconi’s CFA group in 1978 [7]
(60 cm, 4 arc seconds); on ROSAT by J.Trümper at the
Max Plank Institute in 1990 [8]
(80 cm, 5 arc seconds); and on CHANDRA in
1999 (120 cm, 0.5 arc second in the center of a 16x16 arc minutes
field, Figure 1).

Other grazing incidence collectors have used approximations to the
ideal conics optics to obtain varying degrees of concentration of the
X-ray flux, angular resolution and fields of view. XMM-Newton is a Wolter optic
with a modest resolution (~10 arcsec).

The detectors which are used in the focal plane of these telescopes
have been imaging proportional counters, photoelectric high resolution
imaging channeltron devices, and in the last few years charge coupled
devices similar to those used in the optical domain.
The combination of these technical advances has resulted in a 10 order of magnitude
increase in sensitivity from \(3\times 10^{-7} {\rm erg/cm}^{2}/{\rm s}\ ,\) the flux
detected from Sco X-1 (the first source discovered), to the flux of
\(3\times 10^{-17} {\rm erg/cm}^{2}/{\rm s}\) detected with CHANDRA in the deepest
surveys ( Figure 2).

Solar and stellar X-ray emission

Figure 3: Image of the Sun at soft X-ray wavelengths (0.25-4 keV) recorded by the Yohkoh satellite.

Solar emission from the sun occurs because convective currents bring
strong magnetic fields to the surface and thus can heat the coronal
gases to temperatures of 10 million degrees. The magnetic field
contains and accelerates the plasmas as shown by the X-ray
pictures ( Figure 3).
Many other normal stars have
convective zones in their interior and produce X-rays by similar mechanisms.
Recent CHANDRA observations of O supergiant stars have been interpreted as the
result of the interaction of stellar winds in binaries as well as magnetically channeled wind shocks.
The interaction of stellar winds with the interstellar medium may be responsible
for the diffuse emission seen in young star clusters.

Supernova remnants and neutron stars

Neutron stars are the remnants of massive stars which have burned
their elements into iron. The core of the star no longer produces
enough energy to withstand gravitational collapse. The core collapses
until at a density of about \(10^{15} {\rm g/cm}^{3}\) all matter is turned into a
neutron gas which can maintain a stable configuration.

Figure 4: Chandra image of the Crab Nebula showing the central pulsar, the remnant of a supernova seen in 1054 AD, and a complex structure created by high-energy particles spiraling around rings and along jets in perpendicular direction (Credit: NASA/CXC/SAO/J.Hester et al.[1]).

The rotational kinetic energy of the star prior to collapse is
transferred to the neutron star, which rotates very rapidly at its
birth. The magnetic field of the star prior to collapse is also
transferred to the neutron star and is greatly intensified. The
interaction of the rapidly rotating magnetic field with particles can
accelerate these particles to relativistic energies of 1014
eV. These particles produce the radio, optical and X-ray emissions
from the pulsar through synchrotron radiation ( Figure 4).
The energy source is the rotational energy stored in the neutron star
which slowly dissipates resulting in a slowdown of the
rotation. Neutron stars were discovered in 1967 by Hewish and Bell
though their rapid periodic pulsations in radio waves.

The energy dissipated in the stellar collapse drives out the outer
layers of the star in a supernova explosion. Such events were known to
Tycho and Kepler 400 years ago.
The outer layers of the star expand outward at speeds of thousands of kilometers per second,
plow into the interstellar medium and create an expanding shell of hot
gas, at temperatures of million of degrees, which emits strongly in
the X-rays ( Figure 5, U.Hwang et al.).
X-ray spectra of these shells allow us to determine their
metal composition. They expand in the interstellar medium and mix with
it to provide the enriched material required to form systems of stars
and planets similar to our own solar system, capable therefore to
sustain life.

Binary X-ray sources

The discovery of X-ray binaries was crucial in clarifying the nature
of most of the high- luminosity galactic sources. X-ray binaries are
systems composed of a normal star and a collapsed companion, either a
neutron star or a black hole. Cen X-3 and Her X-1 were the first
systems containing a neutron star that were fully understood in terms
of their source of energy and emission processes
(Schreier et al. 1972,
Tananbaum et al. 1972).
Cyg X-1 was the first system that was shown to contain a black hole of a few solar masses.

Cen X-3 and Her X-1 exhibit periodic pulsations with 4.8 and 1.2
second periods. They show occultations and Doppler shifts in the rapid
pulsations in phase with the occultations. The conclusion is that they
are binary systems with 2.1 and 1.7 days period. The compact object is
a neutron star.

The gas from the normal companion falls in the deep potential well of
the compact object and it acquires energy of order of 100 MeV per
nucleon. This energy is transformed in heating of the gas as it
spirals in the accretion disk and as it reaches the surface of the
neutron star. Due to the high temperature the gas is a fully ionized
plasma which is guided to the magnetic poles of the star. As the star
rotates this produces the characteristic periodic pulsations
( Figure 6).
A very small mass loss from the companion is sufficient to
power the X-ray luminosity of 1038 erg/sec some 10,000 times
the total luminosity of the Sun. The accretion of gas onto the neutron star imparts angular
momentum to the star which is gaining rather than loosing rotational
kinetic energy, a process completely opposite to what happens in the
emission from isolated pulsars. The frequency of the rapid pulsations
therefore is increasing rather than decreasing with time.

From these discoveries we learned that if neutron stars are produced
in supernova explosions these explosions do not disrupt the binary
system in which they occur; also that accretion on compact objects
provides a very efficient source of energy, some 50 times more
effective than nuclear burning.

Figure 7: Artist conception of the binary system Cyg X-1 (Illustration of L.Cohen).

In Cyg X-1 a high degree of variability in time scales as short as 1
millisecond was discovered. No long term periodicity was seen. The
optical counterpart was found to be a 5.6 day spectroscopic binary
(Webster and Murdin
and Bolton). The companion star is a super giant
of about 30 solar masses leading to a mass for the compact object of 6
solar masses, much in excess of the limit of 3 solar masses for neutron stars (derived
by Rhoades and Ruffini).
The rapid variability and its large scale sets a constraint of \(3\times 10^7\) cm on the diameter of
the source. Thus the object is comparable in size to a neutron star
but 2 or 3 times more massive. We know of no object with these
characteristics except a black hole, a star predicted by Oppenheimer
and Snyder in 1939. Zeldovich and Novikov had predicted their
discovery through their X-ray emission in 1964
( Figure 7). (See Review in "Accretion-Driven Stellar X-ray Sources")

The very high efficiency for energy production through gravitational
infall provided the physical starting point to explain the high
luminosity of Active Galactic Nuclei and quasi stellar objects as due
to black holes with masses of 106 to 109
solar masses in their centers accreting matter from the galaxy in which they are imbedded.

X-ray emission from galaxies and active galactic nuclei

Normal galaxies emit X-rays due to the sum of the X-ray emission of
main sequence stars, neutron stars, binary X-ray sources, supernova
remnants and diffuse gas. The total X-ray emission is of order
1039 to 1042 erg/sec in the 0.5 to 5 keV range.

Most galaxies (including our own) have massive black holes in their
centers. As these black holes grow by accretion from the surrounding
galaxy their X-ray emission tends to dominate the total emission of the
galaxy. Such objects are called active galaxies (AGNs) or QSOs and
can reach luminosities of 1046 erg/sec. The X-ray
emission is powered by gravitational infall of matter onto the massive black hole at the
center from an accretion disk. Time variability on relatively short
time scales (less than 1 year) is made possible by the relatively
small dimensions of the central engine, and is frequently observed.

A frequent by product of the accretion process is the production of
jets in the direction perpendicular to the plane of the accretion
disk. High energy particles are accelerated to very great distances
(Mpc) from the nucleus. The process of formation of these jets and the
continued acceleration of the particles in them to make up for their
radiative losses is not fully understood ( Figure 8: deep Chandra observations of Cen A by
Hardcastle et al. 2007).

The X-ray background

During the first discovery flight of ScoX-1 in 1962 an isotropic X-ray
background was observed (Giacconi et al. 1962).
This had immediate consequences for
cosmology. Hoyle’s Hot Universe continuous creation theory could not
account for this emission, and this created a major difficulty for the
theory as a whole. Burbidge suggested that the emission could be due
to the sum of unresolved galaxies. With the launch of UHURU in 1970
the existence of the background was confirmed. An upper limit on its
granularity was derived which implied either a diffused emission or a
very large number of individual sources (108 over the entire sky
or one every square arc minute). Woltjer and Setti [9]
suggested that the background could be made up of quasars if they had the same emission
flux as the brightest known nearby quasar 3C273. Giacconi and his
coworkers at Harvard were able to demonstrate in 1979 with data
obtained with the EINSTEIN Observatory that at least 25% of the
background in the 0.5 to 3 keV range was due to single sources,
probably quasars [10].

Some astronomers were not convinced and they pointed out the
resemblance of the spectrum of the background obtained with the HEAO-A
proportional counter in the 3 to 35 kev range to a bremsstrahlung
spectrum, concluding that the XRB was due to a hot gas pervading the
entire universe. This conclusion seemed to be in conflict with the
very large energy requirements to heat this gas for which no source
could be suggested, but it appeared to receive support from the fact
that the spectrum of individual quasars measured with the same
instrument seemed much softer than the background.

The study of active galaxies with ROSAT showed strong evidence that
the in the 0.5 to 3.0 keV range some 80% of the background could be
due to quasars, provided only that the background and quasars spectral
discrepancy could be explained. We believe the work of the University
of Pennsylvania group, led by Niel Brandt, on the CHANDRA deep field
north (CDFN[11]), and that of Giacconi’s group at Johns Hopkins University on
the CHANDRA deep field south (CDFS[12]) have definitively settled the
issue (see [13] for a review).

In our Deep Field South we find a source density of 1 per
square arc minute or 346 sources in 0.1 square degrees
( Figure 9).
The sum of the spectra from the 346 sources equals that of the
background up to ~8 keV. The total flux in point sources in the 0.5 to 5 keV region corresponds
to approximately 95 % of the background, the main uncertainty being
our knowledge of the total background flux. At higher energies, a population of
"Compton thick" AGN, which are obscured by column densities well in excess
of 1024 cm-3 and comprise a significant fraction
of AGN in the local Universe, remains to be unveiled.
Faint sources have spectra harder than that of brighter
ones. The superb angular resolution of Chandra, allowing X-ray
source positional accuracies of better than 1 arcsec, has been critical for an efficient
source identification program with the largest ground-based telescopes
(mainly VLT and Keck), as well as the Hubble and Spitzer Space telescopes.
The sources are all identified in the optical or near
infrared. They consist mainly of Type –I or Type II AGN+QSO. Study of
the variability of these sources yields characteristic times less than
1 year. We conclude that we observe the central massive black hole at
the nucleus of the AGNs. Thus the CDFS and CDFN are fields of
black-holes and the X-ray background radiation is largely the result of accretion
onto super-massive black holes, integrated over cosmic time.
The optical/near-IR identification
and redshift measurement of large samples of AGN in deep pencil-beam surveys, as well as shallower
wide area surveys, has provided a solid determination of the cosmic evolution of their space density at different
luminosities, an essential ingredient for our understanding of the co-evolution of super-massive black holes and galaxies
(see e.g. [14]).

X-ray emission from clusters of galaxies

The discovery of high temperature plasma pervading the space between
galaxies in galaxy clusters has been one of the most important
discoveries of X-ray astronomy (Gursky et al. 1972).
Starting from the UHURU observations of large angular extent, there followed
the determination of the thermal bremsstrahlung nature of the emitted spectrum
with the first detection of plasma iron lines
(Mitchell et al. 1976), the discovery
of structures in the clusters emission and of binary clusters with
EINSTEIN and more and more distant clusters with the ROSAT, CHANDRA
and XMM-Newton Observatories ( Figure 10).

One of the earliest findings was that the X-ray emission from the
clusters was due not only to the sum of the X-ray emission from each
galaxy but also (and prevalently) by the emission of the diffused gas
contained by the gravitational potential of the cluster as a
whole. Since this potential is much greater than that of single
galaxies, the gas could be at much higher temperature (more than 10
keV rather than 1 keV). As the cluster collapses in time due to
gravitational attraction, each particle in the cluster experiences a
gain in energy which results in the heating to these very high
temperatures. The total mass of the intergalactic gas was found to
exceed by factors of 2 to 10 that of all the galaxies contained in the
cluster, thus it played an important role in providing the virial mass
for the cluster, although dark mass was still required for
closure. Because of the large mass (up to 1015 solar masses) and
high temperature, the X-ray emission from the intergalactic gas can exceed by large factors
(10-100) the emission from all stars and galaxies in the cluster.

While the optical luminosity can be written as \(L_{opt}= \sum L_{gal}\ ,\) the
X-ray luminosity is given by \(L_X =\sum L_{X,gal} + \rho_{gas}^2 V_{gas}
T^{1/2}_{gas}\ .\) Thus a gravitationally bound system of galaxies announces
its presence in X-rays by an extended high luminosity source and a bremsstrahlung
spectrum. The presence of emission lines of heavy elements in the
cluster gas, which have been observed out to z=1.3 today, tells us that the gas had to be recycled through the first
generation of stars to be so enriched.

The most recent results from the Chandra observatory have revealed a
complex morphology of the cluster plasma due to cluster-cluster
interactions. A very interesting example is shown in a deep image of
the so-called Bullet cluster obtained by the group at the SAO-Harvard Center for
Astrophysics ( Figure 11).
Comparison between the distribution of dark
matter obtained from optical gravitational lensing and that of the hot
plasma from direct X-ray imaging, show that contrary to what happens
to the plasmas of the colliding clusters, neither the galaxies nor the
dark matter interact strongly in the encounter, an important clue to
the properties of dark matter.

Figure 12: Cumulative number counts of X-ray clusters as a function of the flux, S (from [6]).

Serendipitous cluster surveys, conducted initially with EINSTEIN and more extensively with ROSAT
in the nineties, have determined that the space density of the bulk of the cluster population remains approximately
constant out to redshift unity, while only the abundance of the most massive clusters decreases with redshifts.
The Log N-Log S plot of cluster populations ( Figure 12) shows a continued increase in the numbers at fainter fluxes
(see Rosati et al. 2002 for a review).
Recent observations with XMM-Newton have revealed the existence clusters
at redshifts as large as z=1.4, with masses well in excess of 1014 solar masses, something that
was considered extremely unlikely in the mid-nineties.
The study of these very distant clusters and the evolution of the cluster abundance over a large redshift range can yield
important information on the formation of these systems at early epochs, and tight constraints on cosmological parameters,
including the presence of dark energy which affects the growth of structure in the Universe.